What is Komatsu PC138US-2 Fault Code E108?

Fault Code E108 on the Komatsu PC138US-2 excavator indicates a hydraulic oil temperature sensor circuit malfunction or an abnormal hydraulic oil temperature reading. This diagnostic trouble code (DTC) is triggered when the machine's Electronic Control Module (ECM) detects voltage readings outside the normal operating range from the hydraulic oil temperature sensor, typically located in the hydraulic tank or main pump circuit.

This code is critical for the PC138US-2 because excessive hydraulic oil temperatures can lead to seal deterioration, reduced hydraulic efficiency, and potential system failure. The temperature sensor provides essential data that allows the ECM to protect hydraulic components by alerting operators before damage occurs. In cold weather conditions, this sensor also helps ensure the hydraulics reach proper operating temperature before full-load operation.

Common Symptoms

When Code E108 is active on your Komatsu PC138US-2, operators typically experience:

• Warning light or alarm illuminated on the instrument panel indicating hydraulic system issues
• Erratic hydraulic performance, including inconsistent boom, arm, or bucket response
• Machine derate or reduced power mode where the ECM limits engine RPM to protect the hydraulic system
• Temperature gauge reading shows unusually high, low, or fluctuating hydraulic oil temperatures
• Intermittent code activation, especially during cold starts or after extended operation periods

Potential Causes

The most common technical causes for Code E108 on used PC138US-2 excavators include:

• Faulty hydraulic oil temperature sensor – internal resistance drift or sensor element failure due to prolonged heat exposure
• Damaged wiring harness – particularly at flex points near the hydraulic tank where cables experience vibration and movement
• Corroded or loose electrical connectors – moisture intrusion at the sensor connector is common in older machines
• Short circuit or open circuit in the sensor wiring between the sensor and ECM
• ECM internal fault – less common but possible in high-hour machines with electrical system history
• Actual hydraulic overheating caused by restricted oil cooler, low oil level, or failing hydraulic pump creating excessive heat

How to Troubleshoot and Fix Code E108

Step 1: Visual Inspection Begin by locating the hydraulic oil temperature sensor in the hydraulic tank (typically on the side or top). Inspect the sensor connector for corrosion, oil contamination, bent pins, or loose connections. Check the wiring harness from sensor to ECM for visible damage, wear points where the harness contacts the frame, or evidence of rodent damage. On used excavators, pay special attention to harness routing near moving components.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure its resistance at ambient temperature (consult your service manual for spec, typically 2,000-3,000 ohms at 68°F). Compare readings with Komatsu specifications. Next, check for continuity in the wiring between the sensor connector and ECM connector. Measure voltage supply from the ECM (usually 5V reference voltage) with ignition on, engine off.

Step 3: Component Testing and Replacement If resistance values are out of specification or the sensor shows open or short circuit conditions, replace the hydraulic oil temperature sensor with a genuine Komatsu part. After sensor replacement, clear the code using Komatsu diagnostic software (KOMTRAX) or a compatible scan tool. If wiring shows damage, repair or replace the affected harness section with proper gauge wire and weatherproof connectors. For persistent codes after sensor replacement, ECM reprogramming or replacement may be necessary.

Step 4: System Verification After repairs, monitor actual hydraulic oil temperature during operation and verify it matches the display reading. Run the machine through normal work cycles and confirm the code does not return. Check that hydraulic oil level is correct and oil cooler fins are clean, as actual overheating can trigger this sensor-related code.

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Disclaimer: This guide provides general diagnostic information for the Komatsu PC138US-2. Always consult the official Komatsu service manual for your specific machine serial number and software version. If you're unfamiliar with electrical diagnostics or hydraulic systems, consult a certified Komatsu technician to prevent further damage or safety hazards.